FIG. 1 illustrates an example of a conventional transmission line transformer. In FIG. 1, a reference numeral 101 denotes a primary transmission line and a reference numeral 102 denotes a secondary transmission line of the conventional transmission line transformer. Reference numerals 103 and 104 denote ports connected to the primary transmission line 101, and reference numerals 105 and 106 denote ports connected to the secondary transmission line 102. As shown in a cross-sectional view 109 of the conventional transmission line transformer of FIG. 1, the conventional transmission line transformer formed on an integrated circuit (IC) includes two metal lines on a semiconductor substrate, wherein an insulator is disposed between the metal lines and the semiconductor substrate.
FIG. 2 is a schematic diagram for describing operation principles of the conventional transmission line transformer. A reference numeral 202 of FIG. 2 denotes a direction of a current applied to the primary transmission line 101. When the current is applied to the primary transmission line 101 in the direction 202, a current in a direction 203 is induced in the secondary transmission line 102. Accordingly, the current in the direction 203 opposite to the direction 202 of the current flowing through the primary transmission line 101 is always induced in the secondary transmission line 102. Also, if the conventional transmission line transformer is ideal and thus lossless, a current size in the secondary transmission line 102 is always the same as a current size in the primary transmission line 101.
In the conventional transmission line transformer shown in FIGS. 1 and 2, the primary transmission line 101, the secondary transmission line 102, a metal line 107, and a metal line 108 use a highest layer metal line provided in a corresponding semiconductor process because as a metal line forming a transmission line transformer and a semiconductor substrate are close to each other, a parasitic capacitance component may be generated between the metal line and the semiconductor substrate, and thus a power loss of a signal may be generated on the semiconductor substrate due to a magnetic field generated in the metal line.
A current is induced to the secondary transmission line 102 by a current of the primary transmission line 101 due to a magnetic field formed around the secondary transmission line 102 by the current of the primary transmission line 101. Generally, a coupling factor is used as an index indicating a size of the current induced to the secondary transmission line 102 by the current of the primary transmission line 101. In order to increase the coupling factor, the magnetic field formed by the current of the primary transmission line 101 should largely affect the secondary transmission line 102. Accordingly, as shown in a region 201 of FIG. 2, lengths of the primary transmission line 101 and the secondary transmission line 102 should be long. However, when the lengths of the primary and secondary transmission lines 101 and 102 are increased, a power loss is generated in the transmission line transformer due to the parasitic resistance component caused by metal lines forming the primary and secondary transmission lines 101 and 102. As a result, a length of a metal line forming a transmission line transformer should be increased to increase a coupling factor of the transmission line transformer, but when the length of the metal line is increased, a parasitic resistance component is also increased, thereby causing a power loss.
Accordingly, another conventional transmission line transformer shown in FIG. 3 has been suggested, wherein a parasitic resistance component is decreased by reinforcing a metal line forming the other conventional transmission line transformer, and increasing a coupling factor of the other conventional transmission line transformer. The primary transmission line 101, the secondary transmission line 102, the metal line 107, and the metal line 108 forming the other conventional transmission line transformer of FIG. 3 are formed by using a highest layer metal layer provided in a corresponding semiconductor process. In addition, metal lines 301 and 302 immediately below the highest layer metal line are formed. The metal lines 107 and 301 are electrically connected to each other through a via 303. Similarly, the metal lines 108 and 302 are electrically connected to each other through another via 303. According to the other conventional transmission line transformer of FIG. 3, a coupling factor may be increased as an area of the primary and secondary transmission lines 101 and 102 facing each other is increased while a size of a parasitic resistance component caused by a metal line may be decreased as the primary and secondary transmission lines 101 and 102 are formed by using two layers of metal lines. However, as described above, since the other conventional transmission line transformer of FIG. 3 not only uses a highest layer metal line but also a metal line immediately below the highest layer metal line, a distance between a metal line to which a signal is applied and a semiconductor substrate is decreased, and thus a power loss of a signal on the semiconductor substrate may be higher than that in the conventional transmission line transformer of FIG. 1.